1. Field of the Invention
This invention relates to methods and apparatuses for recording biological signals from living organisms, particularly using bipolar electrodes and Hilbert transforms.
2. Description of Related Art
Nerve and muscle fibers are excitable cells which produce electrical signals propagating along the fibers when excited. The electrical signals propagating along the nerve or muscle fibers are called action potentials (APs). The muscle fibers are organized in groups called motor units (MUs). MUs are the smallest functional groups of muscle fibers innervated by single neurons. When stimulated by a neuron, either all the muscle fibers off a MU will be innervated, or none of them will. The summation of the APs of all the simultaneously activated muscle fibers of a MU is called the MUAP. An electromyographic (EMG) signal is the summation of the repetitive APs of muscle fibers or MUs during muscle contractions. The EMG signals may be used for diagnosis of neuromuscular diseases, investigation of motor control mechanism, and orthotic control. They may be recorded with needle electrodes and, for most superficial muscles, surface electrodes as well. Needle electrodes are inserted into muscle tissue to detect EMG signals, while surface electrodes are placed on the skin above the muscle to detect the signals. Conceptually, surface recording techniques are preferable because they are noninvasive. Nevertheless, needle recording techniques have become the standard and widely used in clinical EMG examinations because surface EMG recording technology had not, prior to this invention, advanced to the point where it could provide as good quality EMG signals as needle recording technology.
Advanced recording technology is required for the use of surface EMG in clinical examinations. What is expected in a surface EMG signal is the detailed information about MUAPs. EMG signals recorded with traditional surface electrodes with large skin-electrode contact area and conventional monopolar or bipolar electrode configurations 10 usually do not contain detailed information about MUAPs. In an effort to develop advanced surface EMG recording methods, a number of avenues have been explored in the literature. Large electrode arrays with more than 50 contacts have been proposed by Masuda and Sadoyamaet [1988] for the investigation of the generation and propagation of the MUAPs. Reucher et al. [1987] suggested pin electrodes with needle points be used as the basic electrodes in an electrode array to reduce the skin-electrode impedance. Reucher et al. also considered spatial filtering techniques which differentiate individual signals obtained with the pin electrodes in the spatial domain to record surface EMG signals selectively. McKingley and Parker [1991] developed a beam former method which improves the signal to noise (S/N) ratio by averaging the signals recorded at different locations along the nerve fibers. In spite of such efforts, no surface EMG recording method has become practical for routine clinical examinations.
In an earlier U.S. patent application. Ser. No. 07/969,458 filed Oct. 30, 1992, Chen et al. describe a practical EMG electrode array. The array is a miniature active electrode array which makes it easy to record multi-channel surface EMG signals. Earlier investigations indicated that the spatial filtering techniques proposed by Reucher et al. are effective in reducing the complexity of the surface EMG signals, but the spatial filters reduce the S/N ratios of the signals significantly.